Loss of pressure on system

[red68]

Can any one help with the following problem on the central heating system?
We have an Alpha CD28C combi boiler, and over the weekend we installed six new radiators.
The system was filled correctly and all the radiators have been bled for air etc,
System works o.k. for hot water and central heating, however after a few hours the red lights “A” and “B” flash alternately and the pressure gauge reading is at low pressure.
We fill the system via the “loop” and all is fine again until a few hours later and then we have to go through the same filling up procedure.
Obviously I thought there must be a leak on the pipework or joints and I have checked all runs of pipe and joints, and there appears to be no loss of water.
The service engineer is adamant that there must be a leak on the system, and he suggests that we would have to loose about a litre of water for the above fault to occur.
As previously stated there are no signs of any water leaks, please could some one advise if the engineer is correct or could there be some other problem with the actual boiler?

 

[Burnerman]

Look for any signs of water dripping from the pressure relief pipe outside.....if the boiler has been overfilled at some time, the pressure relief valves often don't like to fully shut afterwards.
John

 

[Johnmelad]

Is all pipework visible? It is possible to loosen a hidden compression joint when doing other work.

 

[misterdubya]

Turn the boiler off.
Pressurise to 1.2 bar (ish).
Isolate the flow & return pipes that run into the boiler. (Valves under the boiler).
Leave for a few hours - do not use in this period.
If pressure remains the same after several hours, the leak is on the system, if it has fallen then it is on the boiler.
Open up the isolation valves, and continue to use.